1. Field of the Invention
The present invention relates to a power transmission system such as a speed change gear for a four-wheel drive.
2. Description of the Prior Art
A clutch mechanism of a power transmisssion system for a vehicle which changes over drive states, for example, between the two-wheel drive state and the four-wheel drive state, has a clutch which effects engagement and disengagement between two shafts: a central shaft which drives wheels on one side of the vehicle; and an outer peripheral shaft which transmits power to wheels on the other side. The clutch cylinder of this clutch has heretofore been supported on the outer periphery of the outer peripheral shaft, which fact disadvantageously makes it difficult for a subsidiary transmission to be reduced in its axial direction and unfavorably complicates the arrangement of an oil line for supplying working oil to a hydraulic servo provided in the hydraulic cylinder.
Further, a conventional power transmission system for a vehicle has, as shown in FIG. 5, a casing 509 which is composed of a transmission casing 503, an input-side casing 505a, a front casing 505b, a front sprocket casing 506, a rear sprocket casing 507 and an extension housing 508. The transmission casing 503 is provided therein with a main transmission mechanism. The input-side casing 505a is fastened to the transmission casing 503 and receives an input shaft 520 of a transfer 510 which constitutes the power transmission system, the input shaft 520 serving as the output shaft of the main transmission mechanism and having a governor valve 510a secured thereto. The input-side casing 505a further receives a first output shaft 581 of the transfer 510 which is disposed at the rear (rightwardly as viewed in FIG. 5) of and coaxially with the input shaft 520, and a second output shaft 561 which is disposed in parallel to the first output shaft 581. Furthermore, the input-side casing 505a houses a hydraulically-actuate clutch C.sub.3 actuated by a hydraulic servo C-3 which is constituted by a cylinder 553 and a piston 554 received therein. In addition, the input-side casing 505a has an oil pan 592 fastened to the lower side thereof. The front casing 505b contains a planetary gear transmission mechanism 505A having a planetary gear set Pf and a hydraulically-actuated brake B4 which is actuated by a hydraulic servo B-4. The planetary gear set Pf is constituted by a sun gear S.sub.1, a planetary pinion P, a ring gear R.sub.1 and a carrier P.sub.1, while the hydraulic servo B-4 is constituted by a cylinder 551 defined by a portion of an intermediate support wall which supports the first output shaft 581 and a piston 552 received in the cylinder 551. The front sprocket casing 506 contains a mechanism 506A which changes over drive states between the two-wheel drive and the four-wheel drive and the front portion of each of the sprockets 566 and 575. The mechanism 506A is constituted by a hydraulically-actuated clutch C.sub.4 which is actuated by a hydraulic servo C-4 constituted by a cylinder 563 and a piston 564 received therein. The rear sprocket casing 507 houses a link mechanism 576 which is constituted by the sprocket 566 connected to a sleeve 567, the sprocket 575 connected to the second output shaft 561 and a chain 574 stretched between these sprockets 566 and 575. The extension housing 508 houses a speedometer drive gear 583. Thus, the conventional power transmission system enables changeover between the two-wheel drive, the high-speed four-wheel drive and the low-speed four-wheel drive during the traveling of the vehicle by virtue of the combination of the planetary gear set Pf, the clutches C.sub.3, C.sub.4 and the brake B.sub.4.
The transfer 510 serving as a power transmission system for a vehicle and arranged as above, however, suffers from the following disadvantage. Namely, since the cylinder 551 of the hydraulic servo B-4 for the brake B.sub.4 is integrally formed with the front casing 505b and is employed as a casing for the purpose of preventing any damage to transfer elements when they are mounted and the casings are bolted to each other, the number of sections of the casing 509 is disadvantageously increased, which fact involves a fear of working oil leaking out from the casing 509 of the power transmission system.